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Mars rover sees signs of microbe-friendly layers in ancient lake

Once there was water

NASA/JPL-Caltech/MSSS

By Leah Crane

Ancient Mars’s lakes may have had two habitable zones. A study of mudstones in Gale crater shows that the lake that once resided there was split into distinct layers: one closer to the water’s edges and surface, and another deeper down. Both had everything a microbe needs to survive.

In August 2012, the Curiosity rover landed in Gale crater. Since then, it has been trekking towards the mountain at the crater’s centre. The rover has travelled more than 16 kilometres in about 1700 Martian days (each of which is just longer than an Earth day), taking measurements of the rocks it passes.

Now, Joel Hurowitz at Stony Brook University, New York, and his colleagues have used measurements from Curiosity’s first 1300 Martian days to characterise the lake that filled the crater more than 3 billion years ago.

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They found that many iron deposits near the edges of the crater are rusty, indicating that the water near the lake’s surface was rich in oxidants, compounds that force other substances to lose their electrons.

Samples taken towards the middle of the lakebed weren’t oxidised. The iron there probably arrived in groundwater seeping into the lake from below, as opposed to the deposits at the edges, which were carried in by rivers.

“Oftentimes, lakes on Earth become chemically stratified in this same way,” says Hurowitz. “It doesn’t feel very alien or unfamiliar.”

Ripe for life

This so-called oxidation gradient, along with previous reports of a variety of minerals and organic compounds in the area, indicates that the lake could have been ripe for life – both the types that need oxidants and those that don’t.

“Not only was this lake a very habitable environment on Mars, but there were multiple sub-environments within that lake,” says Hurowitz. “If microbial life was present on Mars at that time, there would have been multiple niches for those microbes to pick and choose from.”

The two zones within the lake would have been affected by the strength of the ultraviolet light hitting it, as well as the amount of oxygen in the atmosphere, so researchers can use the layered rocks on the lakebed to learn about the ancient Martian climate beyond the crater. Their models show that, in the hundreds of thousands to millions of years that the lake was stable, the area around it was slowly warming.

“Gale crater is one location, and the difficulty that we have with Curiosity is separating out the history of this one location and the history of every other location,” says Bruce Jakosky at the University of Colorado at Boulder.

Even as the lakebed rocks show rising temperatures over cosmically short periods of time, the planet’s overall climate was beginning to cool and dry out. “This detailed information may help us figure out how these lakes fit into the broader environmental and hydrological cycles,” says Jakosky.

The new findings add to previous evidence that ancient Mars had an environment hospitable to life, with all the water, chemicals and energy sources organisms would need.

“The Gale crater landing site is proving to be one of the most interesting and valuable places on the planet to learn about its history and its capacity to support biological activity,” says Nicholas Tosca at the University of Oxford.